Lynn has it right. Combine that with finding out how much force until they fail (male or female, big or small) while still using proper posture. Once the posture fails under load (that could be just them obviously straining) then lighten up. For shugyo, keep it just on that edge. Always push a little more each keiko, so they keep improving under greater loads.

Wash, rinse, repeat, make accomodations for bad days, etc.

Best,
Ron

Ron Tisdale
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"The higher a monkey climbs, the more you see of his behind."
St. Bonaventure (ca. 1221-1274)